Imaging devices having piezoelectric transceivers with harmonic characteristics

ABSTRACT

A micromachined ultrasonic transducer (MUT) which comprises a first piezoelectric layer and a second piezoelectric layer. The first piezoelectric layer is disposed between a first electrode and a second electrode. The second piezoelectric layer is disposed between the second electrode and a third electrode. At least the first electrode has first and second ends along a first axis, one or more of which is defined by a radius of curvature R. A second axis normal to the first passes through a midpoint of the first axis. A half-width of the first electrode is defined by a length L measured from the midpoint, in the direction of the second axis, to an outer perimeter of the first electrode. A total width of the first electrode at its narrowest point along the first axis is at most 2L such that the first electrode has a concave shape. R/L, is greater than 1.

CROSS-REFERENCE

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 17/218,656, filed Mar. 31, 2021, and is also acontinuation-in-part of PCT Application No. PCT/US2021/025109, filedMar. 31, 2021, the contents of which are fully incorporated herein byreference.

The subject matter of this application is related to that of: U.S.patent application Ser. No. 16/833,333; U.S. patent application Ser. No.16/837,800; U.S. Pat. Nos. 10,656,007; 10,648,852; and U.S. applicationSer. No. 17/218,656, the contents of which are fully incorporated hereinby reference.

BACKGROUND Technical Field

The present invention relates to imaging devices and, more particularly,to imaging devices having micromachined ultrasound transducers (MUTs)that exhibit enhanced pressure amplitude and frequency response behaviorwhen driven at fundamental and harmonic frequencies.

Background

A non-intrusive imaging system for imaging internal organs of a humanbody and displaying images of the internal organs transmits signals intothe human body and receives signals reflected from the organs.Typically, transducers, such as capacitive transducers (cMUTs) orpiezoelectric transducers (pMUTs), that are used in an imaging systemare referred to as transceivers and some of the transceivers are basedon photo-acoustic or ultrasonic effects.

In general, a MUT includes two or more electrodes and the topology ofthe electrodes affects both electrical and acoustic performances of theMUT. For instance, the amplitude of acoustic pressure generated by apMUT increases as the size of the electrodes increase, to therebyimprove the acoustic performance of the pMUT. However, as the size ofthe electrodes increase, the capacitance also increases to degrade theelectrical performance of the pMUT. In another example, the amplitude ofacoustic pressure at a vibrational resonance frequency of the pMUT isaffected by the shape of the electrodes. As such, there is a need formethods for designing electrodes to enhance both acoustical andelectrical performances of the transducers.

SUMMARY

In an aspect, a micromachined ultrasonic transducer (MUT) is disclosed.The MUT comprises at least a first piezoelectric layer and a secondpiezoelectric layer. The first piezoelectric layer is disposed between afirst electrode and a second electrode. The second piezoelectric layeris disposed between the second electrode and a third electrode. At leastthe first electrode has first and second ends along a first axis. One ormore of the first end or second end is defined by a radius of curvatureR. A second axis passes through a midpoint of the first axis, whereinthe second axis is normal to the first axis. A half-width of the firstelectrode is defined by a length L measured from the midpoint, in thedirection of the second axis, to an outer perimeter of the firstelectrode. A total width of the first electrode at its narrowest pointalong the first axis is at most 2L such that the first electrode has aconcave shape. R/L, is greater than 1.

In some embodiments, the first axis extends along a direction where thefirst electrode has a longest dimension.

In some embodiments, the second axis extends along a direction where thefirst electrode has a shortest dimension.

In some embodiments, the piezoelectric layer is formed of at least oneof PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO₃.

In an aspect, a micromachined ultrasonic transducer (MUT) is disclosed.The MUT comprises a plurality of piezoelectric layers comprising Mpiezoelectric layers. The MUT also comprises a plurality of electrodescomprising N electrodes. A piezoelectric layer of the plurality ofpiezoelectric layers with an index m is disposed between a firstelectrode with an index m and a second electrode with an index m+1. Theindex m is associated with a vertical distance of the piezoelectriclayer. At least the first electrode having first and second ends along afirst axis. One or more of the first end or second end is defined by aradius of curvature R. A second axis passes through a midpoint of thefirst axis. The second axis is normal to the first axis. A half-width ofthe first electrode is defined by a length L measured from the midpoint,in the direction of the second axis, to an outer perimeter of the firstelectrode. A total width of the first electrode at its narrowest pointalong the first axis is at most 2L such that the first electrode has aconcave shape. R/L, is greater than 1.

In some embodiments, the first axis extends along a direction where thefirst electrode has a longest dimension.

In some embodiments, the second axis extends along a direction where thefirst electrode has a shortest dimension.

In some embodiments, the MUT further comprises a substrate and amembrane suspending from the substrate.

In some embodiments, the piezoelectric layer is formed of at least oneof PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO₃.

In some embodiments, N=M+1.

In an aspect, a micromachined ultrasonic transducer (MUT) is disclosed.The MUT comprises at least a first piezoelectric layer and a secondpiezoelectric layer. The first piezoelectric layer is disposed between afirst electrode and a second electrode. The second piezoelectric layeris disposed between the second electrode and a third electrode. At leastthe first electrode has first and second ends along a first axis. One ormore of the first end or second end is defined by a radius of curvatureR. A second axis passes through a midpoint of the first axis, whereinthe second axis is normal to the first axis. A half-width of the firstelectrode is defined by a length L measured from the midpoint, in thedirection of the second axis, to an outer perimeter of the firstelectrode. A total width of the first electrode at its widest pointalong the first axis is at least two times L such that the firstelectrode has a convex shape. R/L, is less than 1.

In some embodiments, the first axis extends along a direction where thefirst electrode has a longest dimension.

In some embodiments, the second axis extends along a direction where thefirst electrode has a shortest dimension.

In some embodiments, the MUT further comprises a substrate and amembrane suspending from the substrate.

In some embodiments, the piezoelectric layer is formed of at least oneof PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO₃.

In an aspect, a micromachined ultrasonic transducer (MUT) is disclosed.The MUT comprises a plurality of piezoelectric layers comprising Mpiezoelectric layers. The MUT also comprises a plurality of electrodescomprising N electrodes. A piezoelectric layer of the plurality ofpiezoelectric layers with an index m is disposed between a firstelectrode with an index m and a second electrode with an index m+1. Theindex m is associated with a vertical distance of the piezoelectriclayer. At least the first electrode having first and second ends along afirst axis. One or more of the first end or second end is defined by aradius of curvature R. A second axis passes through a midpoint of thefirst axis. The second axis is normal to the first axis. A half-width ofthe first electrode is defined by a length L measured from the midpoint,in the direction of the second axis, to an outer perimeter of the firstelectrode. A total width of the first electrode at its widest pointalong the first axis is at least two times L such that the firstelectrode has a convex shape. R/L, is less than 1.

In some embodiments, the first axis extends along a direction where thefirst electrode has a longest dimension.

In some embodiments, the second axis extends along a direction where thefirst electrode has a shortest dimension.

In some embodiments, the MUT further comprises a substrate and amembrane suspending from the substrate.

In some embodiments, the piezoelectric layer is formed of at least oneof PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO₃.

In some embodiments, N=M+1.

In embodiments, a micromachined ultrasonic transducer (MUT) includes atop electrode. The shape of the top electrode is defined by a major andminor axis, where the major and minor axis intersect at an origin point.Both distal ends of the top electrode, i.e., the ends of the topelectrode farthest from the origin in the direction of the major axis,are defined by radius of curvature, R. The characteristic width of thetop electrode, L, is measured from the origin, in the direction of theminor axis (i.e., normal to the major axis), to the top electrode outeredge or perimeter. When the ratio of the radius of curvature over thecharacteristic width, R/L, is more than one, the top electrode is widerat its ends as compared to its width at the middle, and the electrodehas a generally concave geometry. When the ratio of the radius ofcurvature over the characteristic width, R/L, is less than one, the topelectrode is narrower at its ends as compared to its width at themiddle, and the electrode has a generally convex geometry. As set forthin greater detail herein, whether configured with either concave orconvex geometry, electrodes with certain R/L, values or within certainvalue ranges exhibit desirable pressure amplitude and frequency responsebehavior when driven at fundamental and harmonic frequencies, relativeto prior electrode shape designs. The areal density distribution of theconcave or convex electrode along an axis has a plurality of localmaxima, wherein locations of the plurality of local maxima coincide withlocations where a plurality of anti-nodal points at a vibrationalresonance frequency are located.

In embodiments, a micromachined ultrasonic transducer (MUT) includes asymmetric convex top electrode. The areal density distribution of thesymmetric convex electrode along an axis has a plurality of localmaxima, wherein locations of the plurality of local maxima coincide withlocations where a plurality of anti-nodal points at a vibrationalresonance frequency are located.

In embodiments, a transducer array includes a plurality of micromachinedultrasonic transducers (MUTs). Each of the plurality of MUTs includes asymmetric convex top electrode.

In embodiments, an imaging device includes a transducer array that has aplurality of micromachined ultrasonic transducers (MUTs). Each of theplurality of MUTs includes a symmetric convex top electrode. The arealdensity distribution of the symmetric convex electrode along an axis hasa plurality of local maxima and wherein locations of the plurality oflocal maxima coincide with locations where a plurality of anti-nodalpoints at a vibrational resonance frequency are located.

In embodiments, a micromachined ultrasonic transducer (MUT) includes asymmetric concave top electrode. The areal density distribution of thesymmetric concave electrode along an axis has a plurality of localmaxima, wherein locations of the plurality of local maxima coincide withlocations where a plurality of anti-nodal points at a vibrationalresonance frequency are located.

In embodiments, a transducer array includes a plurality of micromachinedultrasonic transducers (MUTs). Each of the plurality of MUTs includes asymmetric concave top electrode.

In embodiments, an imaging device includes a transducer array that has aplurality of micromachined ultrasonic transducers (MUTs). Each of theplurality of MUTs includes a symmetric concave top electrode. The arealdensity distribution of the symmetric concave electrode along an axishas a plurality of local maxima and wherein locations of the pluralityof local maxima coincide with locations where a plurality of anti-nodalpoints at a vibrational resonance frequency are located.

In a first aspect, a micromachined ultrasonic transducer (MUT) isprovided. The MUT comprises a first electrode having first and secondends along a first axis. One or more of the first end or second end isdefined by a radius of curvature R. A second axis passes through amidpoint of the first axis, wherein the second axis is normal to thefirst axis. A half-width of the first electrode is defined by a length Lmeasured from the midpoint, in the direction of the second axis, to anouter perimeter of the first electrode. A total width of the firstelectrode at its widest point along the first axis is at least two timesL such that the first electrode has a convex shape and R/L, is less than1.

In embodiments, the MUT is a capacitive micromachined ultrasoundtransducer (cMUT).

In embodiments, the MUT is a piezoelectric micromachined ultrasoundtransducer (pMUT).

In embodiments, the first axis extends along a direction where the firstelectrode has a longest dimension.

In embodiments, the second axis extends along a direction where thefirst electrode has a shortest dimension.

In embodiments, the MUT further comprises a substrate; a membranesuspending from the substrate; a second electrode disposed on themembrane; and a piezoelectric layer disposed on one or more of the firstelectrode or the second electrode. In some embodiments, thepiezoelectric layer comprises a first piezoelectric layer disposed onthe second electrode. In some embodiments, the MUT further comprises athird electrode disposed on the first piezoelectric layer; and a secondpiezoelectric layer disposed on the third electrode, wherein the firstelectrode is disposed on the second piezoelectric layer. In embodiments,the piezoelectric layer is formed of at least one of PZT, KNN, PZT-N,PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO₃.

In another aspect, an imaging device is provided. The imaging devicecomprises a transducer array including a plurality of micromachinedultrasonic transducers (MUTs), each of the plurality of MUTs comprisinga convex electrode.

In another aspect, an MUT is provided. The MUT comprises a firstelectrode having first and second ends along a first axis. One or moreof the first end or second end is defined by a radius of curvature R. Asecond axis passes through a midpoint of the first axis, wherein thesecond axis is normal to the first axis. A half-width of the firstelectrode is defined by a length L measured from the midpoint, in thedirection of the second axis, to an outer perimeter of the firstelectrode. A total width of the first electrode at its narrowest pointalong the first axis is less than 2L such that the first electrode has aconcave shape and R/L is greater than 1.

In embodiments, the MUT is a capacitive micromachined ultrasoundtransducer (cMUT).

In embodiments, the MUT is a piezoelectric micromachined ultrasoundtransducer (pMUT).

In embodiments, the first axis extends along a direction where the firstelectrode has a longest dimension.

In embodiments, the second axis extends along a direction where thefirst electrode has a shortest dimension.

In embodiments, the MUT further comprises a substrate; a membranesuspending from the substrate; a second electrode disposed on themembrane; and a piezoelectric layer disposed on one or more of the firstelectrode or the second electrode. In some embodiments, thepiezoelectric layer comprises a first piezoelectric layer disposed onthe second electrode. In some embodiments, the MUT further comprises athird electrode disposed on the first piezoelectric layer; and a secondpiezoelectric layer disposed on the third electrode, wherein the firstelectrode is disposed on the second piezoelectric layer. In embodiments,the piezoelectric layer is formed of at least one of PZT, KNN, PZT-N,PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO₃.

In another aspect, an imaging device is provided. The imaging devicecomprises a transducer array including a plurality of micromachinedultrasonic transducers (MUTs), each of the plurality of MUTs comprisinga concave electrode.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

References will be made to embodiments of the invention, examples ofwhich may be illustrated in the accompanying figures. These figures areintended to be illustrative, not limiting. Although the invention isgenerally described in the context of these embodiments, it should beunderstood that it is not intended to limit the scope of the inventionto these particular embodiments.

FIG. 1 shows an imaging system according to embodiments of the presentdisclosure.

FIG. 2 shows a schematic diagram of an imager according to embodimentsof the present disclosure.

FIG. 3A shows a side view of a transceiver array according toembodiments of the present disclosure.

FIG. 3B shows a top view of a transceiver tile according to embodimentsof the present disclosure.

FIG. 4A shows a cross sectional view of a MUT, applicable to a concaveor a convex MUT, taken along a direction 4-4 in FIG. 4B and FIG. 4D,according to embodiments of the present disclosure.

FIG. 4B shows a top view of a concave MUT according to embodiments ofthe present disclosure.

FIG. 4C shows an alternative top view of a concave MUT according toembodiments of the present disclosure.

FIG. 4D shows a top view of a convex MUT according to embodiments of thepresent disclosure.

FIG. 4E shows an alternative top view of a convex MUT according toembodiments of the present disclosure.

FIG. 4F shows a cross sectional view of another MUT, applicable to aconcave or a convex MUT, taken along a direction 4-4 in FIG. 4B and FIG.4D, according to embodiments of the present disclosure.

FIG. 4G illustrates a top view of a MUT with multiple piezoelectriclayers.

FIG. 4H illustrates a sub-section of the top view of FIG. 4G displayedso that the radius R and characteristic half-width are visible.

FIG. 5A shows a plot of acoustic responses of a MUT having a concaveconfiguration according to embodiments of the present disclosure.

FIG. 5B shows a plot of acoustic responses of a MUT having a convexconfiguration according to embodiments of the present disclosure.

FIGS. 6A-6C show vibrational mode shapes of concave and convex MUTsaccording to embodiments of the present disclosure.

FIG. 7A illustrates a plot of the frequency response of a dual-layerpMUT, compared to frequency responses of single-layer pMUT devices.

FIG. 7B illustrates a plot of the frequency response of a dual-layerpMUT that has electrodes that are concave in shape, compared to thefrequency response of a dual-layer pMUT that has electrodes that areconvex in shape.

FIG. 7C illustrates a plot of the frequency response of a dual-layerpMUT that has electrodes that are convex in shape, compared to thefrequency response of a dual-layer pMUT that has electrodes that areconvex in shape.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, specificdetails are set forth in order to provide an understanding of thedisclosure. It will be apparent, however, to one skilled in the art thatthe disclosure can be practiced without these details. Furthermore, oneskilled in the art will recognize that embodiments of the presentdisclosure, described below, may be implemented in a variety of ways,such as a process, an apparatus, a system, or a device.

Elements/components shown in diagrams are illustrative of exemplaryembodiments of the disclosure and are meant to avoid obscuring thedisclosure. Reference in the specification to “one embodiment,”“preferred embodiment,” “an embodiment,” or “embodiments” means that aparticular feature, structure, characteristic, or function described inconnection with the embodiment is included in at least one embodiment ofthe disclosure and may be in more than one embodiment. The appearancesof the phrases “in one embodiment,” “in an embodiment,” or “inembodiments” in various places in the specification are not necessarilyall referring to the same embodiment or embodiments. The terms“include,” “including,” “comprise,” and “comprising” shall be understoodto be open terms and any lists that follow are examples and not meant tobe limited to the listed items. Any headings used herein are fororganizational purposes only and shall not be used to limit the scope ofthe description or the claims. Furthermore, the use of certain terms invarious places in the specification is for illustration and should notbe construed as limiting.

Overview

Disclosed is a multilayer micromachined ultrasonic transducer (MUT). Themultilayer MUT may include multiple alternating piezoelectric layers andelectrodes (thus being a multilayer pMUT), stacked on top of oneanother, in a manner such that the piezoelectric layers are “sandwiched”between sets of electrodes. Thus, a given electrode that is not the toplayer or the bottom layer may be shared by two piezoelectric layers,being disposed on above a first piezoelectric layer and below a secondpiezoelectric layer.

Although specific MUT devices disclosed herein are dual-layer,comprising two piezoelectric layers and three conducting layers, orelectrodes, MUT devices with additional layers may be designed. As thenumber of layers increases, the performance of the device may improve.But if the number of layers is too large, the capacitance within thedevice may increase. As a result, the rise time of signal input may slowdown, degrading performance. Generally, a MUT device with Mpiezoelectric layers may have N conducting layers, where N would equalM+1.

A piezoelectric transducer may transmit acoustic waves into a medium,and receive acoustic waves from the medium, converting the receivedwaves into electrical signals. Piezoelectric materials accumulateelectrical charge when mechanically stressed. Piezoelectric materialsalso exhibit a reverse piezoelectric effect. When an electric field isapplied to a piezoelectric material, the applied field causes amechanical strain. Thus, when a piezoelectric transducer is driven in atransmit (Tx) mode by applying an alternating current (AC) voltage, theoscillating voltage causes the piezoelectric layer to vibrate, producingan acoustic wave. This wave may resonate within a substrate layerdisposed under the piezoelectric layer, increasing the output power ofthe acoustic wave prior to transmission. In a receive (Rx) mode, areflected acoustic wave causes a mechanical stress on the piezoelectriclayer, causing it to accumulate charge. The accumulated charge maygenerate an electrical signal that may be read by an imaging device.

Augmenting a MUT transducer with additional piezoelectric layers mayincrease the power of a generated acoustic wave by producing morevibrations when stressed by an AC voltage and may accumulate more chargein response to a stress from a received acoustic wave. Thus, suchdevices may be both more powerful and more sensitive than single-layerdevices.

A device with multiple piezoelectric layers may thus generate acousticwaves with larger output power. As more vibrations are produced when avoltage is applied to a multilayer pMUT, more powerful sound waves maybe produced. Conversely, an incident wave may stress the multiplepiezoelectric layers, generating a larger signal.

In addition to implementing additional piezoelectric layers, thedisclosed MUT device may use convex or concave electrodes that areshaped to exhibit desirable pressure amplitude and frequency responses,when driven at fundamental frequencies and harmonics.

The disclosure defines a MUT aspect ratio, an engineering parameter thatmay provide information about improved performance at high frequencies.The aspect ratio may be calculated by dividing the length of a long sideof an electrode by twice its characteristic half-width (an electrode'sshort side width at its midpoint). Thus, for electrodes of the samesize, a convex electrode may have a smaller aspect ratio than a concaveelectrode.

This disclosure suggests multilayer MUTs with particular aspect ratiosmay exhibit increased acoustic wave power, and thus better performance,at high frequencies. A multilayer concave pMUT disclosed herein with alarger aspect ratio than another concave pMUT was shown to have betterperformance at high frequencies. This result accords with a generalrelationship between increased R/L, in concave pMUTs and improvedhigh-frequency performance.

Multilayer MUT Device

Disclosed herein is a micromachined ultrasonic transducer (MUT), whichmay be a piezoelectric MUT (pMUT device).

The MUT device may comprise at least two piezoelectric layers. In someembodiments, the MUT device may comprise at least 3 layers, at least 4layers, at least 5 layers, at least 6 layers, at least 10 layers. TheMUT device may comprise at most 3 layers, or at most 4 layers, or atmost 5 layers, or at most 6 layers, or at most 10 layers. The MUT devicemay comprise between 3 and 6 layers, or between 4 and 7 layers, orbetween 5 and 8 layers, or between 6 and 9 layers, or between 7 and 10layers. The piezoelectric layers may be of uniform thickness. Thepiezoelectric layers may be of non-uniform thickness. The piezoelectriclayers may all be of different thicknesses. Subsets of piezoelectriclayers may have the same thickness but may be distinct in thickness fromother subsets.

A piezoelectric layer may be disposed between two electrodes, a secondelectrode above and a first electrode below. Each piezoelectric layer inthe device may be disposed between two electrodes in this manner. Thus,the device may include one more electrode than there are piezoelectriclayers, as layers may share electrodes. In some embodiments, the MUTdevice may comprise at least 3 electrodes, at least 4 electrodes, atleast 5 electrodes, at least 6 electrodes, at least 10 electrodes. TheMUT device may comprise at most 3 electrodes, or at most 4 electrodes,or at most 5 electrodes, or at most 6 electrodes, or at most 10electrodes. The MUT device may comprise between 3 and 6 electrodes, orbetween 4 and 7 electrodes, or between 5 and 8 electrodes, or between 6and 9 electrodes, or between 7 and 10 electrodes, or between 8 and 11electrodes.

In some embodiments, at least the first electrode has first and secondends along a first axis. In some embodiments, all of the electrodes havefirst and second ends along the first axis. In other embodiments, lessthan half the electrodes have first and second ends along the firstaxis. In other embodiments, more than half, but fewer than all, of theelectrodes have first and second ends along the first axis.

One or more of the first end of an electrode or the electrodes or asecond end of the electrode or electrodes may be defined by a radius ofcurvature R. The first end and the second end may be shaped likeportions of circles. The first and second ends may or may not have thesame radius of curvature R. The first ends and/or second mayadditionally be shaped like portions of ovals, triangles, squares, orrectangles, but may not employ sharp edges, which may degradeperformance of the MUT. The first ends and/or second ends may be shapeddifferently. The electrodes may all have the same shape. Some electrodesof the multi-electrode MUT may have the same shape, and others may be ofanother shape. Some or all of the electrodes may each be of differentshapes.

The first and second axis may be normal or perpendicular to one another.The second axis may pass through a midpoint of the first axis, and/orvice-versa.

The multilayer MUT may include electrodes that have particular shapes toimprove performance. The MUT electrode may be substantially ovular oroblong, with a long side and a short side. The width of the electrodemay vary along an axis bisecting the long side of the electrode. Forexample, close to the midpoint, the electrode may be thinner than on thelong ends, creating a concave shape. In other embodiments, the electrodemay be thicker in the middle than on the long ends, creating a convexshape.

A half-width of an electrode may be defined by a length L measured fromthe midpoint, in the direction of the second (or short) axis, to anouter perimeter of the electrode. All of the electrodes may have thesame L. One set of electrodes may have the same L, while the otherelectrodes may have a different value for L. Additionally, some or allof the electrodes may have multiple different values of L.

In some embodiments, the total width of an electrode at its narrowestpoint along the first axis is less than 2L such that the electrode has aconcave shape. The electrode may be one of the first, second, third,fourth, fifth, sixth, seventh, eighth, ninth, tenth, or eleventhelectrode. The electrode may be more than one, but less than all, of thefirst, second, third, fourth, fifth, sixth, seventh, eighth, ninth,tenth, or eleventh electrodes. All of the electrodes may have the sameconcave shape, with their total widths at their narrowest points alongtheir first axes being less than 2L.

In some embodiments, the total width of an electrode at its widest pointalong the first axis is at least two times L such that the electrode hasa convex shape. The electrode may be one of the first, second, third,fourth, fifth, sixth, seventh, eighth, ninth, tenth, or eleventhelectrodes. The electrode may be more than one, but less than all, ofthe first, second, third, fourth, fifth, sixth, seventh, eighth, ninth,tenth, or eleventh electrodes. All of the electrodes may have the sameconvex shape, with their total widths at their widest points along theirfirst axes being greater than two times L In some embodiments, allelectrodes may be symmetrical. In some embodiments, one or moreelectrodes may be asymmetrical with respect to the first axis and/or thesecond axis.

In embodiments where an electrode has a convex shape, the ratio of R toL may be less than 1. For example, the ratio may be less than 0.01, lessthan 0.1, less than 0.2, less than 0.3, less than 0.4, less than 0.5,less than 0.6, less than 0.7, less than 0.8, less than 0.9, less than0.99, or less than 0.999. The ratio may be greater than about 0.01,greater than 0.1, greater than 0.2, greater than 0.3, greater than 0.4,greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8,greater than 0.9, greater than 0.99, or greater than 0.999. The ratiomay be between 0 and 0.1, between 0.1 and 0.2, between 0.2 and 0.3,between 0.3 and 0.4, between 0.4 and 0.5, between 0.5 and 0.6, between0.6 and 0.7, between 0.7 and 0.8, between 0.8 and 0.9, and between 0.9and 1.

In embodiments where the electrode has a concave shape, the ratio of Rto L may be greater than 1. For example, the ratio may be less than1.01, less than 1.1, less than 1.2, less than 1.3, less than 1.4, lessthan 1.5, less than 2, less than 5, less than 10, less than 25, lessthan 50, or less than 100. The ratio may be greater than about 1.01,greater than 1.1, greater than 1.2, greater than 1.3, greater than 1.4,greater than 1.5, greater than 2, greater than 5, greater than 10,greater than 25, greater than 50, or greater than 100. The ratio may bebetween 1.0 and 1.1, between 1.1 and 1.2, between 1 and 2, between 2 and5, between 5 and 10, between 10 and 25, between 25 and 50, or between 50and 100.

In some embodiments, the MUT also includes a substrate and a membranesuspending from the substrate. The substrate may comprise asemiconducting material or may comprise multiple alternating layers ofsemiconducting materials and insulating materials. For example, thesubstrate may comprise a silicon layer. The substrate may also comprisealternating layers of silicon and silicon dioxide (i.e., asilicon-on-insulator (SOI) substrate). In other embodiments, thesubstrate may comprise semiconducting materials such as germanium,silicon-germanium, carbon-doped silicon, carbon-doped silicon-germanium,or another material. In some embodiments, the substrate may include acavity disposed underneath the MUT's alternating piezoelectric layersand electrodes.

In some embodiments, the membrane may be a silicon layer configured toassist with transmitting and receiving acoustic waves. The membrane mayvibrate in response to motion from the piezoelectric elements, after anAC voltage is applied to them, which may in turn may produce theacoustic wave that is transmitted to the subject. When an acoustic waveis reflected, the acoustic wave may disturb the membrane, in turnstressing the piezoelectric layers and causing them to produceelectrical signals. These electrical signals may be provided to animaging device. a

In embodiments of the disclosure, the first axis extends along theelectrode's longest dimension. In embodiments of the disclosure, thesecond axis extends along the electrode's shortest dimension.

In embodiments of the disclosure, at least one piezoelectric layer maybe formed of at least one of PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO,PVDF, and LiNiO₃. In some embodiments, all of the piezoelectricmaterials are formed of the same material. In some embodiments, at leastone piezoelectric layer is a composite of one of the aforementionedmaterials. In some of the embodiments, a first plurality ofpiezoelectric layers may be formed from one material, and a secondplurality may be formed from another material. In some embodiments, allpiezoelectric layers are heterogeneous. In some embodiments, a firstplurality of piezoelectric layers are homogeneous, and a secondplurality of piezoelectric layers are heterogeneous.

In embodiments of the disclosure, a MUT device may have M piezoelectriclayers. The MUT device may have N electrodes. As a piezoelectric layermay be “sandwiched” between two electrodes, the number N may be equal toM+1. Generally, for an M-piezoelectric layer MUT device, one may indexthe piezoelectric layers from 1 to M, where 1 is the layer closest tothe substrate and M is the layer furthest from the substrate. For apiezoelectric layer m in this set, adjacent electrodes may be indexed asm and m+1, for electrodes closer to and further away from the substrate,respectively. For example, a second piezoelectric layer may be providedan index of 2, where the electrodes below and above it may be indexed 2and 3, respectively.

Description of the Figures

FIG. 1 shows a schematic diagram of an imaging system 100 according toembodiments of the present disclosure. As depicted, the system 100 mayinclude: an imager 120 that generates and transmits pressure waves 122toward an internal organ 112, such as heart, in a transmit mode/processand receives pressure waves reflected from the internal organ; and adevice 102 that sends and receives signals to the imager through acommunication channel 130. In embodiments, the internal organ 112 mayreflect a portion of the pressure waves 122 toward the imager 120, andthe imager 120 may capture the reflected pressure waves and generateelectrical signals in a receive mode/process. The imager 120 maycommunicate electrical signals to the device 102 and the device 102 maydisplay images of the organ or target on a display/screen 104 using theelectrical signals.

In embodiments, the imager 120 may be used to get an image of internalorgans of an animal, too. The imager 120 may also be used to determinedirection and velocity of blood flow in arteries and veins as in Dopplermode imaging and also measure tissue stiffness. In embodiments, thepressure wave 122 may be acoustic waves that can travel through thehuman/animal body and be reflected by the internal organs, tissue orarteries and veins.

In embodiments, the imager 120 may be a portable device and communicatesignals through the communication channel 130, either wirelessly (usinga protocol, such as 802.11 protocol) or via a cable (such as USB2, USB3, USB 3.1, USB-C, and USB thunderbolt), with the device 102. Inembodiments, the device 102 may be a mobile device, such as cell phoneor iPad, or a stationary computing device that can display images to auser.

In embodiments, more than one imager may be used to develop an image ofthe target organ. For instance, the first imager may send the pressurewaves toward the target organ while the second imager may receive thepressure waves reflected from the target organ and develop electricalcharges in response to the received waves.

FIG. 2 shows a schematic diagram of the imager 120 according toembodiments of the present disclosure. In embodiments, the imager 120may be an ultrasonic imager. As depicted in FIG. 2 , the imager 120 mayinclude: a transceiver tile(s) 210 for transmitting and receivingpressure waves; a coating layer(s) 212 that operate as a lens forsetting the propagation direction of and/or focusing the pressure wavesand also functions as an acoustic impedance interface between thetransceiver tile and the human body 110; a control unit 202, such asASIC chip (or, shortly ASIC), for controlling the transceiver tile(s)210 and coupled to the transducer tile 210 by bumps; Field ProgrammableGate Arrays (FPGAs) 214 for controlling the components of the imager120; a circuit(s) 215, such as Analogue Front End (AFE), forprocessing/conditioning signals; an acoustic absorber layer 203 forabsorbing waves that are generated by the transducer tiles 210 andpropagate toward the circuit 215; a communication unit 208 forcommunicating data with an external device, such as the device 102,through one or more ports 216; a memory 218 for storing data; a battery206 for providing electrical power to the components of the imager; andoptionally a display 217 for displaying images of the target organs.

In embodiments, the device 102 may have a display/screen. In such acase, the display may not be included in the imager 120. In embodiments,the imager 120 may receive electrical power from the device 102 throughone of the ports 216. In such a case, the imager 120 may not include thebattery 206. It is noted that one or more of the components of theimager 120 may be combined into one integral electrical element.Likewise, each component of the imager 120 may be implemented in one ormore electrical elements.

In embodiments, the user may apply gel on the skin of the human body 110before the body 110 makes a direct contact with the coating layer 212 sothat the impedance matching at the interface between the coating layer212 and the human body 110 may be improved, i.e., the loss of thepressure wave 122 at the interface is reduced and the loss of thereflected wave travelling toward the imager 120 is also reduced at theinterface. In embodiments, the transceiver tiles 210 may be mounted on asubstrate and may be attached to an acoustic absorber layer. This layerabsorbs any ultrasonic signals that are emitted in the reversedirection, which may otherwise be reflected and interfere with thequality of the image.

As discussed below, the coating layer 212 may be only a flat matchinglayer just to maximize transmission of acoustic signals from thetransducer to the body and vice versa. Beam focus is not required inthis case, because it can be electronically implemented in control unit202. The imager 120 may use the reflected signal to create an image ofthe organ 112 and results may be displayed on a screen in a variety offormat, such as graphs, plots, and statistics shown with or without theimages of the organ 112.

In embodiments, the control unit 202, such as ASIC, may be assembled asone unit together with the transceiver tiles. In other embodiments, thecontrol unit 202 may be located outside the imager 120 and electricallycoupled to the transceiver tile 210 via a cable. In embodiments, theimager 120 may include a housing that encloses the components 202-215and a heat dissipation mechanism for dissipating heat energy generatedby the components.

FIG. 3A shows a side view of a transceiver array 200 according toembodiments of the present disclosure. FIG. 3B shows a top view of atransceiver tile 210 according to embodiments of the present disclosure.In embodiments, the array 200 may include one or more transceiver tiles210. As depicted, the transceiver array 200 may include one or moretransceiver tiles 210 arranged in a predetermined manner. For instance,as depicted in FIG. 3A, the transceiver tiles (or, shortly tiles) 210may be physically bent to further form a curved transceiver array anddisposed in the imager 120. It should be apparent to those of ordinaryskill in the art that the imager 120 may include any suitable number oftiles and the tiles may be arranged in any suitable manner, and eachtile 210 may include any suitable number of piezoelectric elements 302having a concave or convex shape as described in greater detail herein,that are disposed on a transceiver substrate 304. On the substrate 304,one or multiple number of temperature sensors 320 may be placed in orderto monitor the temperature of the transceiver tile 210 during operation.In embodiments, the transceiver array 200 may be a micro-machined arrayfabricated from a substrate.

FIG. 4A shows a cross sectional view of a MUT 400, according toembodiments of the present disclosure. The cross sectional view of FIG.4A is applicable to a concave or a convex MUT, according to embodimentsof the present disclosure. As depicted, the concave or convex MUT mayinclude: a membrane layer 406 suspended from a substrate 402; a first(e.g., bottom) electrode (O) 408 disposed on the membrane layer (or,shortly membrane) 406; a piezoelectric layer 410 disposed on the bottomelectrode (O) 408; and a second (e.g., top) electrode (X) 412 disposedon the piezoelectric layer 410.

In embodiments, the substrate 402 and the membrane 406 may be onemonolithic body and the cavity 404 may be formed to define the membrane406. In embodiments, the cavity 404 may be filled with a gas at apredetermined pressure or an acoustic damping material to control thevibration of the membrane 406. In embodiments, the geometrical shape ofthe projection area of the top electrode 412 may be configured in agenerally concave or convex shape having characteristic geometricparameters to control the dynamic performance and capacitance magnitudeof the MUT 400.

In embodiments, each MUT 400 may by a pMUT and include a piezoelectriclayer formed of at least one of PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN,ZnO, PVDF, and LiNiO₃. In alternative embodiments, each MUT 400 may be acMUT.

In embodiments, each MUT 400 may include additional electrodes and/orPZE layers. For example, as shown in FIG. 4F, the MUT 400 (be itconcave, convex, or otherwise shaped as desired) may include: a membranelayer 406 suspended from a substrate 402; a first electrode (O) 408disposed on the membrane layer (or, shortly membrane) 406; a firstpiezoelectric layer 410 disposed on the first electrode (O) 408; asecond electrode 414 disposed on the first piezoelectric layer 410; asecond piezoelectric layer 410 disposed on the second electrode 414; anda third electrode (X) 412 disposed on the second piezoelectric layer410. Additional piezoelectric layers 410 and electrodes may be added asdesired. In at least some instances, adding additional piezoelectriclayers and/or electrodes (i.e., “sandwiching” electrodes andpiezoelectric layers) increases the amplitude/dB output of the MUT 400.

In FIGS. 4B-4E and 4G-H, each MUT 400 is shown to have either a concaveor convex shape. In embodiments, each concave MUT may include a topelectrode that has a concave shape when viewed from the top of the MUT400. In embodiments, each convex MUT may include a top electrode thathas a convex shape when viewed from the top of the MUT 400. Hereinafter,the term shape of the top electrode 412 refers to a two-dimensionalshape of the top electrode obtained by projecting the top electrode onto the x-y plane. Also, the shape of the top electrode is calledsymmetric if the shape is symmetric with respect to the two lines 450and 452, where the lines 450 and 452 are parallel to the x- and y-axes,respectively, and pass through the midpoint of the top electrode on thex-axis. Also, hereinafter, the x-axis, also referred to herein as themajor axis, extends along the direction where the top electrode has thelongest dimension. The y-axis, also referred to herein as the minoraxis, extends along the direction normal to the x- or major axis in thex-y plane, along the direction where the top electrode has the shortestdimension.

Whether the concave MUT of FIGS. 4B-4C and 4G-H or the convex MUT ofFIGS. 4D-4E, the shape of the top electrode is defined by a major andminor axis, where the major and minor axis intersect at an origin point.Both distal ends of the top electrode, i.e., the ends of the topelectrode farthest from the origin in the direction of the major axis,also referred to herein as the “heads” of the electrode, are defined bya radius of curvature, R. The characteristic half-width of the topelectrode, also referred to herein as the “foot” of the electrode, isdefined by a length L measured from the origin, in the direction of theminor axis (i.e., normal to the major axis), to the electrode outer edgeor perimeter. The total width of the electrode at either its narrowestpoint (if a concave MUT) or its widest point (if a convex MUT) is twotimes L (i.e., 2L). Edges of the electrode between the head and foot ofthe electrode may be curved or straight.

Alternatively, the heads of the electrodes may be straight or be definedby other curvature geometries that are not entirely circular. In atleast some instances, it may be beneficial to avoid) geometries thatcreate localized areas of concentrated mechanical stress, which mayenable a local mechanical or material failure mode, such as might becreated if the head (or foot, for example) were defined by two straightlines converging at a sharp point.

In some embodiments, the head need not be circular, but may also bedefined by, without limitation, non-circular curvature such as that of aparabola. Whereas a circular electrode head might be defined by a radiusof curvature, R, the relevant parameter for a parabolic-shaped headmight be the semilatus rectum of the parabola, defined as twice thedistance between the parabola focus and vertex. Furthermore, theperimeter between the head and the foot may be defined either by astraight line or curvature and still be within the scope of thisinvention.

As illustrated in FIGS. 4B-4C and 4G-4H, when the ratio of the radius ofcurvature over the characteristic width, R/L, is more than one, the topelectrode is wider at its ends, or heads, as compared to its width atthe middle, or foot, and the electrode has a generally concave geometry.As one of ordinary skill in the art would appreciate, variations of thisconcave MUT geometry are possible and still within the scope of thisdisclosure by varying the R/L, ratio, so long as R/L, is more than one,i.e., R>L. For example, for a top electrode having a head radius ofcurvature, R, of 43 micrometers, suitable foot width, L, of betweenapproximately 37 and 41 micrometers would fall within the scope of thisdisclosure and exhibit the enhanced pressure amplitude and frequencyresponse behavior when driven at fundamental and harmonic frequencies,as will be further described herein. On the other hand, if R/L, is toolarge, whereby the head radius or curvature R is much larger than thefoot width, L, the top electrode may not exhibit the desired pressureamplitude and frequency response behavior when driven at fundamental andharmonic frequencies, and can even experience structural failure.

As illustrated in FIGS. 4D-4E, when the ratio of the radius of curvatureover the characteristic width, R/L, is less than one, the top electrodeis narrower at its ends as compared to its width at the middle, and theelectrode has a generally convex geometry. As one of ordinary skill inthe art would appreciate, variations of this convex MUT geometry arepossible and still within the scope of this disclosure by varying theR/L ratio, so long as R/L is less than one, i.e., R<L. For example, fora top electrode having a head radius of curvature, R, of 43 micrometers,suitable foot width, L, of between approximately 43.1 and 500micrometers would fall within the scope of this disclosure and exhibitthe enhanced pressure amplitude and frequency response behavior whendriven at fundamental and harmonic frequencies, as will be furtherdescribed herein. On the other hand, if R/L is too small, whereby thehead radius or curvature R is much smaller than the foot width, L, thetop electrode suffers from undesirable pressure amplitude and frequencyresponse behavior when driven at fundamental and harmonic frequencies,and can even experience structural failure.

Whether configured with either concave or convex geometry, electrodeswith certain R/L values or within certain R/L, value ranges exhibitdesirable pressure amplitude and frequency response behavior when drivenat fundamental and harmonic frequencies, relative to certain priorelectrode shape designs. The areal density distribution of the concaveor convex electrode along an axis has a plurality of local maxima,wherein locations of the plurality of local maxima coincide withlocations where a plurality of anti-nodal points at a vibrationalresonance frequency are located. In general, the acoustic pressureperformance, which refers to the energy of an acoustic pressure wavegenerated by each MUT at a frequency, may increase as the peak amplitudeof the MUT increases at the frequency.

The ratio or R/L, may be driven by the desired behavior of the MUT.Changing the R/L, parameter of the electrode (and therefore, thegeometry of the electrode) varies the pressure amplitude and frequencyresponse behavior of the electrode. R/L may be large or small withoutlimitation, so long as the electrode exhibits the desired pressureamplitude and/or frequency response behavior. The design requirements ofthe particular transducer, which may be dictated by factors such astransducer end use case (e.g., industrial, medical diagnosis, etc.),power requirements, operating mode requirements, etc., inform whetherthe pressure amplitude and frequency response exhibited by a particularR/L geometry is acceptable or desirable. Additional considerations suchas fabrication and material capabilities may further limit the desirableor available range acceptable R/L, ranges.

FIG. 4G illustrates a top view of a MUT with multiple piezoelectriclayers. The top view shows the electrode which is the furthest (e.g.,the largest vertical or z-distance) from the substrate of the MUTdevice. When viewed from the top (e.g., above the MUT, wherein the MUTis in a negative z-direction from the viewer), the top MUT electrode hasa concave shape. In this embodiment, all of the electrodes may have thesame shape, as layers of electrodes underneath the top electrode arehidden from view. In this embodiment, at least the top electrode issymmetric with respect to the x- and y-axes. In other embodiments,electrodes disposed beneath the top electrode may have different sizes.In other embodiments, electrodes disposed beneath the top electrode mayhave different shapes.

In assessing whether an electrode design may yield improved performanceat high frequencies, this disclosure defines an electrode aspect ratio.The aspect ratio parameter is related to the ratio R/L and is defined asthe ratio of the length of the electrode (the length from the tip of thetop head to the base of the bottom head) to twice the characteristichalf-width (or two times L). With respect to the embodiments disclosedherein, the concave embodiment of FIG. 4G has aspect ratio 7, the convexembodiment of FIG. 4D has aspect ratio 3, and the concave embodiment ofFIG. 4B has aspect ratio 5. Generally, a larger aspect ratio maycorrelate with increased performance at high frequencies.

When an electrode has a characteristic half-width L that is equal to itsradius, it may be stadium-shaped rather than convex or concave. Anelectrode of given length would have a higher aspect ratio as a concaveelectrode, a medium aspect ratio as a stadium-shaped electrode, and alower aspect ratio as a convex electrode. The use of the aspect ratiosuggests that a longer convex electrode, or set of electrodes, may beable to achieve similar performance to a shorter concave electrode.Conversely, a smaller concave electrode may be able to be used in placeof a larger convex electrode.

FIG. 4H illustrates a sub-section of the top view of FIG. 4G displayedso that the radius R and characteristic half-width L are visible. As inFIG. 4C, the value of R is larger than that of L.

FIGS. 5A-5B show exemplary idealized plots 500 and 510 of acousticresponses of a MUT having a concave configuration and a MUT having aconvex configuration, according to embodiments of the presentdisclosure. FIG. 5A shows an idealized plot 500 of how acoustic powerchanges with frequency for a concave MUT 504 (e.g., R/L>1) compared to aMUT 502 with R/L=1. FIG. 5B shows an idealized plot 510 of how acousticpower changes with frequency for a convex MUT 514 (e.g., R/L<1) comparedto a MUT 512 with R/L=1. For concave MUT 504, as indicated by arrow 506,as R/L increases, the power-frequency curve shifts to the right comparedto MUT 502. For convex MUT 514, as indicated by arrow 516, as R/L,decreases, the power-frequency curve shifts to the upwards compared toMUT 512.

Further modifications to the concave or convex MUT geometry, includingvarying the thickness of the membrane (e.g., a silicon membrane), oradding single or double notches at the periphery of the membrane (suchthat the membrane behaves more like a pinned beam or spring, rather thana cantilevered beam) may provide further enhanced performancecharacteristics. Examples of such modifications can be found in U.S.patent application Ser. Nos. 17/018,304 and 15/820,319, which areincorporated herein by reference.

FIGS. 6A-6C show three vibrational modes 600, 610, 620, according toembodiments of the present disclosure. In FIGS. 6A-6C, each of the MUTs602, 612, and 622, whether concave or convex, is represented by a singleline for the purpose of illustration, where each single line shows thecurvature of the stack of layers in a MUT. During operation, the stackof layers having the membrane 406, bottom electrode 408, piezoelectriclayer 410, and top electrode 412 may move as a single body in thevertical direction, and may be deformed to have the curvature of thesingle line on the x-z plane. Also, the lines 602, 612, and 622, thatcorrespond to different vibrational modes show the curvatures of thestack at different vibrational modes. In general, the resonancecharacteristics of a concave MUT and a convex MUT are similar to oneanother, though local gain may change depending on whether the MUT isconcave or convex. In some instances, selection of a convex geometry ora concave geometry may be driven the gain improvements achieved atcertain frequencies of interest.

In embodiments, the three vibrational modes 600, 610, and 620 may beassociated with three vibrational resonance frequencies, f1, f2, and f3,respectively. In FIGS. 6A-6C, only three vibrational modes are shown.However, it should be apparent to those of ordinary skill in the artthat a concave or convex MUT may operate in more than three vibrationalresonance modes (or shortly vibrational modes).

In FIG. 6A, the concave or convex MUT 602 may operate in the firstvibrational mode 600, where the arrow 604 indicates that the MUT 602(more specifically, the stack of layers) moves in the vertical directionin the first mode 600. In embodiments, the first vibrational mode 600may be symmetric, i.e., the mode shape is symmetric with respect to thecenterline 606 of the MUT. In embodiments, the shape of the topelectrode of the MUT 602 may be symmetric and either be concave orconvex, as shown in FIGS. 4B-4E.

In FIG. 6B, the MUT 612 may operate in the second vibrational mode 610.In embodiments, the second vibrational mode 610 may be symmetric, i.e.,the mode shape is symmetric with respect to the centerline 606.Hereinafter, the term symmetric vibrational mode refers to a vibrationalmode where the locations of the anti-nodal points, such as 615, 616, and617, (i.e., the peak amplitudes) are arranged symmetrically with respectto a centerline 606, and the centerline 606 represents a line that isparallel to the z-axis and passes through the midpoint of the MUT on thex-axis.

In the second vibrational mode 610, the MUT 612 may have two nodalpoints and three anti-nodal points (or equivalently, three peakamplitude points) 615, 616, and 617. In embodiments, the shape of thetop electrode of the MUT 612 may be symmetric and either be concave orconvex, as shown in FIGS. 4B-4E.

In FIG. 6C, the MUT 622 may operate in the third vibrational mode 620.In embodiments, the third vibrational mode 620 may be symmetric, i.e.,the mode shape is symmetric with respect to the centerline 606. In thethird vibrational mode, the MUT 622 may have four nodal points and fiveanti-nodal points (i.e. five peak amplitude points) 624, 625, 626, 627,and 628. In embodiments, the shape of the top electrode of the MUT 622may be symmetric and either be concave or convex, as shown in FIGS.4B-4E.

In general, the acoustic pressure performance, which refers to theenergy of an acoustic pressure wave generated by each MUT at afrequency, may increase as the peak amplitude of the MUT increases atthe frequency. However, relative to a convex MUT of same or similartotal area, the concave MUT has greater local area distributions at thedistal ends compared to the middle (i.e., R>L). As a consequence,relative to a convex MUT of same or similar area, the concave MUT isable to output higher acoustic pressure amplitude, particularly atharmonic frequencies.

It is noted that each of the MUTs 302 in FIG. 3 may be a piezoelectricmicromachined ultrasound transducer (pMUT). However, it should beapparent to those of ordinary skill in the art that the transceiver tile210 may include an array of capacitive micromachined ultrasoundtransducers (cMUTs), i.e., the piezoelectric elements 302 may bereplaced by cMUTs. In such a case, the top electrode of a CMUT may havea shape that is similar to one of shapes of the top electrodes 412, sothat the acoustic response of the cMUT is controlled at variousvibrational resonance frequencies, based on the principles described inconjunction with FIGS. 4B-6C.

FIG. 7A illustrates a plot of the frequency response 710 of a dual-layerpMUT, compared to frequency responses 720 of single-layer pMUT devices.At high frequencies (between about 5 and 10 megahertz (MHz)), thedual-layer pMUT produces acoustic waves with greater acoustic power (indB) than do the single piezoelectric layer pMUTs.

FIG. 7B illustrates a plot of the frequency response 740 of a dual-layerpMUT that has electrodes that are concave in shape, compared to thefrequency response 750 of a dual-layer pMUT that has electrodes that areconvex in shape. As can be seen from the plot, the pMUT with concaveelectrodes outperforms the pMUT with convex electrodes, generatingacoustic waves with more power in a high-frequency region of interest760 between about 4.5 MHz and 8 MHz.

FIG. 7C illustrates a plot of the frequency response 770 of a dual-layerpMUT that has electrodes that are convex in shape, compared to thefrequency response 760 of a single-layer pMUT that has electrodes thatare convex in shape. As can be seen from the plot, the dual-layer pMUToutperforms the single-layer pMUT at most frequencies of operation.While the invention is susceptible to various modifications andalternative forms, specific examples thereof have been shown in thedrawings and are herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formsdisclosed, but to the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling within the scope ofthe appended claims.

What is claimed is:
 1. A micromachined ultrasonic transducer (MUT),comprising: at least a first piezoelectric layer and a secondpiezoelectric layer, wherein the first piezoelectric layer is disposedbetween a first electrode and a second electrode, wherein the secondpiezoelectric layer is disposed between the second electrode and a thirdelectrode, and when seen in a top view, the first electrode having firstand second end portions, a longitudinal axis, and a transverse axisextending perpendicular relative to the longitudinal axis through amidpoint of the first electrode, the midpoint having a longitudinallocation along the longitudinal axis, the first and second end portionscomprising semi-circular shapes each having a constant radius ofcurvature R, the first electrode being symmetric about both thelongitudinal and transverse axes and defining an outer perimeter havinglongitudinal edges that taper toward the midpoint such that the shapehas a narrowest transverse width at the longitudinal location of themidpoint, a half-width of the first electrode being defined by a lengthL measured from the midpoint, along the transverse axis, to the outerperimeter of the first electrode, the narrowest transverse width beingless than 2R such that the longitudinal edges of the first electrodecurve inward in a concave curve, and wherein R/L is greater than
 1. 2.The MUT of claim 1, wherein the longitudinal axis extends along adirection where the first electrode has a longest dimension.
 3. The MUTof claim 1, wherein the transverse axis extends along a direction wherethe first electrode has a shortest dimension.
 4. The MUT of claim 1,further comprising: a substrate; and a membrane suspending from thesubstrate.
 5. The MUT of claim 4, wherein the piezoelectric layer isformed of at least one of PZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO,PVDF, and LiNiO₃.
 6. The MUT of claim 1, wherein the first end or secondend of the first electrode comprises a non-circular curved geometry. 7.The MUT of claim 1, wherein the first, second, and third electrodes havethe same shape.
 8. The MUT of claim 1, wherein the first, second, andthird electrodes have different shapes.
 9. The MUT of claim 1, whereinthe first, second, and third electrodes are symmetrical with respect toat least one of the first and second axes.
 10. The MUT of claim 1,wherein at least one of the first, second, and third electrodes areasymmetrical with respect to at least one of the first and second axes.11. A micromachined ultrasonic transducer (MUT), comprising: a pluralityof piezoelectric layers comprising M piezoelectric layers; and aplurality of electrodes comprising N electrodes, wherein a piezoelectriclayer of the plurality of piezoelectric layers with an index m isdisposed between a first electrode with an index m and a secondelectrode with an index m+1, wherein the index m is associated with avertical distance of the piezoelectric layer, and when seen in a topview, the first electrode having first and second end portions, alongitudinal axis, and a transverse axis extending perpendicularrelative to the longitudinal axis through a midpoint of the firstelectrode, the midpoint having a longitudinal location along thelongitudinal axis, the first and second end portions comprisingsemi-circular shapes each having a constant radius of curvature R, thefirst electrode being symmetric about both the longitudinal andtransverse axes and defining an outer perimeter having longitudinaledges that taper toward the midpoint such that the shape has a narrowesttransverse width at the longitudinal location of the midpoint, ahalf-width of the first electrode being defined by a length L measuredfrom the midpoint, along the transverse axis, to the outer perimeter ofthe first electrode, the narrowest transverse width being less than 2Rsuch that the longitudinal edges of the first electrode curve inward ina concave curve, and wherein R/L is greater than
 1. 12. The MUT of claim11, wherein the first axis extends along a direction where the firstelectrode has a longest dimension.
 13. The MUT of claim 11, wherein thesecond axis extends along a direction where the first electrode has ashortest dimension.
 14. The MUT of claim 11, further comprising: asubstrate; and a membrane suspending from the substrate.
 15. The MUT ofclaim 11, wherein the piezoelectric layer is formed of at least one ofPZT, KNN, PZT-N, PMN-Pt, AIN, Sc-AIN, ZnO, PVDF, and LiNiO₃.
 16. The MUTof claim 11, wherein N=M+1.
 17. The MUT of claim 11, wherein the firstend or second end of the first electrode comprises a non-circular curvedgeometry.
 18. The MUT of claim 11, wherein the first, second, and thirdelectrodes have the same shape.
 19. The MUT of claim 11, wherein thefirst, second, and third electrodes have different shapes.
 20. The MUTof claim 11, wherein the first, second, and third electrodes aresymmetric with respect to at least one of the first and second axes. 21.The MUT of claim 11, wherein at least one of the first, second, andthird electrodes are asymmetrical with respect to at least one of thefirst and second axes.